This invention relates to commercial chlor-alkali cells which produce chlorine gas, hydrogen gas, and aqueous sodium hydroxide by the electrolysis of aqueous sodium chloride. More particularly this invention relates to a cathode assembly which can be used with low electrical power (or voltage) requirements in a vertical, finger (or pocket) interleaved anode and cathode electrolytics cells which utilize ion exchange membranes to separate the anodes and cathodes.
Many advances have been made in recent years in the improvement of chlor-alkali production cells. For example, cell voltages have been reduced and cell life prolonged by the replacement of graphite blade anodes with ruthenium oxide coated titanium in the highly corrosive anolyte chambers of these cells. Also higher purity of the aqueous sodium hydroxide has been made possible by the replacement of asbestos type diaphragms with the ion-exchange membranes such as Nafion.RTM.. Many other improvements have been made as time has passed, but obviously the tremendous capital costs tied up in existing chlor-alkali plants can not be discarded each time an advance is made. Hence, these improvements have, for the most part, been incorporated into existing cells by adjusting the improvements to fit the existing facilities rather than abandoning existing plants and redesigning and building totally new plants each time an improvement is made. Thus, many inventions have been made by discovering ways to modify new discoveries to work with existing technology. An example of this type of invention is found in Loftfield, U.S. Pat. No. 3,591,483 (July 6, 1971), wherein metal anodes were made with extra width so as to be as wide as the graphite blades they were to replace without wasting excess metal, and thus avoid extensive redesigning of the cells in use.
In the chlor-alkali cell field there are types of diaphragm cells wherein the cathode is commonly referred to as a vertical pocket or a vertical finger. In these cells the cathode is made of a pair of vertical, foraminous, metal members which are connected together in spaced apart relation at their top, bottom, and leading edge to form a hollow interior, or "pocket". This assemblage is attached at its open vertical back edges to a vertical backscreen (or backplate) which has openings therethrough to allow cathodic products to escape out the back of the cathode pocket as opposed to escaping through the top of the pocket. This type of cathode design is what is meant by "vertical" cathode pockets in this specification. Examples of such vertical cathode pockets can be found in several patents, e.g., Cunningham et al, U.S. Pat. No. 4,016,064 (Apr. 5, 1977), Hunter, et al., U.S. Pat. No. 2,282,058 (Jan. 24, 1939).
With the advent of ion-exchange membranes to replace the diaphram in existing vertical type cells it has been discovered that modification can be made in the vertical cathode to enable the cells to operate at higher power efficiencies through reduced cell voltage.
This and other benefits and advantages have been achieved in the present invention as will become apparent in the following description and examples.